Lecture 26 - Fluid Forces

Question 1

what is a fluid?

Answer 1

• liquid or gas
• anything that will fill the container it is in

Question 2

what is drag force?

Answer 2

• when we move through a fluid, the fluid applies a drag force
• drag force slows our motion down
• we must split the fluid and close it behind us which takes energy (drag force)
• drag opposes the motion (like friction)

Question 3

what reduces drag force in sports?

Answer 3

• when someone in front of you is breaking the fluid, you can stay in the space behind and use less energy
• used by cyclists and nascar

Question 4

what is the equation for calculating drag force?

Answer 4

• Fdrag = 1/2 pv^2CdragA
• Fdrag = is drag force
• p (roe) is density of air
• Cdrag = drag of coefficient of the projectile
• A = cross-sectional area

Question 5

how is drag coefficient affected by streamlining?

Answer 5

• drag coefficient is smaller with streamlining

Question 6

what is the cross-sectional area?

Answer 6

• the area of the structure ‘presented to the flow’
• can make this smaller by bringing their arms in

Question 7

what is relative velocity?

Answer 7

• the velocity between the fluid and the object

Question 8

compare the drag and velocity of head wind vs tail wind

Answer 8

• head wind = anterior wind, against the direction of movement, larger relative velocity with greater drag (ex: 20+5 = 25 m/sec)
• tail wind = posterior wind, assisting the direction of movement, smaller relative velocity with less drag (ex: 20-5 = 15 m/sec)

Question 9

what is sidewind and relative velocity?

Answer 9

• fluid is moving to the side (alongside head/tail wind)
• lateral component to the fluid velocity that the object fells
• drag force is angled relative to the objects pathway
• ex: plane landing in a storm

Question 10

what is drag force direction?

Answer 10

• the drag force direction changes during the flight since the direction of the velocity of the object changes with its parabolic path
• ex: as the ball goes up, drag force pulls down, as the ball falls down, drag force pushes up

Question 11

where does drag force come from? (what are the two sources)

Answer 11

• skin drag and profile drag

Question 12

what is skin drag?

Answer 12

• the friction between the surface of the object and the fluid as it passes over
• the fluid immediately next to the surface is in fact stuck to the surface and not moving

Question 13

what is profile drag?

Answer 13

• the drag caused by the separation of the boundary layer from a surface and the wake created by that separation
• when an object “splits” the fluid, and the fluid has to reconnect
• the lower pressure behind the object that “sucks” the object backwards

Question 14

what is the boundary layer?

Answer 14

• the space between the non-moving surface layer and the undisturbed fluid flow
• this is opposed by the viscosity of the fluid
• gets thin and viscous in water/honey

Question 15

what is laminar flow?

Answer 15

• when viscosity is really high or moving really slow

Question 16

how does viscosity affect skin drag?

Answer 16

• greater viscosity has greater skin drag (aka water > honey)

Question 17

why does a golf ball have dimples?

Answer 17

• make the boundary level more turbulent (narrower)
• reduces skin drag and allows the ball to travel further since its drag is less (makes a thinner wake)

Question 18

how does region size affect profile drag?

Answer 18

• smaller region = less profile drag
• larger region = higher profile drag

Question 19

what is the angle of attack?

Answer 19

• the angle at which the object approaches the fluid flow
• larger angle of attack = larger profile drag

Question 20

what is streamlining?

Answer 20

• get rid of the wake (fill the back with material)
• reduces the pressure pull in the back and helps reconnect the fluid flow

Question 21

how is streamlining beneficial to profile drag?

Answer 21

• must be oriented properly with respect to the fluid flow
• otherwise can actually increase profile drag

Question 22

what is lift force?

Answer 22

• a force that acts at a right angle to drag
• arises due to Bernoulli’s principle (faster fluid flow creates lower pressure)
• lower pressure ‘sucks’ on the object
• perpendicular to relative flow direction and drag force

Question 23

what is the equation for lift force?

Answer 23

F lift = 1/2 pv^2C lift A
- C lift = life coefficient

Question 24

how does angle of attack influence lift force?

Answer 24

• greater angle of attack = greater lift
• angle of attack is the angle between direction of flow and discus

Question 25

what is stalling?

Answer 25

• too large of an angle of attack causes the drag to be too large and therefore makes the object stall

Question 26

what is the lift to drag ratio?

Answer 26

• high enough angle to get lift, but low enough angle to get limited drag